Every gardener has experienced it, usually more times than they can count. You walk into the garden and discover a plant that’s been defoliated or otherwise damaged by caterpillars. The canna leaves are riddled with holes, the cabbage leaves look like lace, half the tomatoes have worms in the fruit, or the azaleas have been stripped of their leaves. How could this happen so quickly?

Caterpillars are the immature stage of moths and butterflies. Although most gardeners enjoy seeing butterflies and moths in their garden, they feel quite differently about caterpillars. Butterflies and moths are beautiful insects that feed on nectar and do not damage plants, but many caterpillars are voracious pests of vegetable and ornamental plants. Heavy caterpillar infestations can completely defoliate, or even kill, prized plants.

Azalea caterpillars are common defoliators of azaleas, and sometimes blueberries, especially in the more southern areas of the Southeast. (2 inches)

Caterpillars are sometimes characterized as crawling stomachs. From the time they hatch from the egg until they pupate, eating is their primary occupation. They have to take periodic breaks to molt, or shed their skin, but they soon resume feeding. This is why caterpillars are such damaging pests. Depending on species, even one caterpillar can cause a lot of damage, but many moths can lay hundreds of eggs per night.

Fortunately, not all caterpillars are pests. Caterpillars that only feed on weeds or other undesirable plants are not pests and are sometimes considered beneficial. Despite their huge appetites, most caterpillars are picky eaters and will only feed on a relatively narrow range of plants. Tobacco hornworms feed on tomatoes as well as tobacco, peppers and other solanaceous plants, but they won’t eat the leaves of oaks or most other plants. Conversely, pink-striped oakworms love oak leaves but will starve rather than eat tomato leaves. Monarch butterfly caterpillars are one of the few insects that can survive on milkweed, and milkweeds are the only plants they will eat. Secondary chemicals that occur in different groups of plants are one of the key reasons for this host specificity. Compare the odor of crushed tomato leaves to that of broccoli and rosemary and you will get a whiff of some of these chemicals.

Tomato fruitworms usually bore in near the stem end of the tomato. One caterpillar can destroy several fruit, a heartbreaking experience for serious tomato growers. (1.25 inches)

There are a few species of caterpillars that have unusually wide host ranges and these tend to be some of our most important pest species. Tomato fruitworm, Helicoverpa zea, is one of the best examples. This pest actually has three official common names. In tomatoes it is called tomato fruitworm, in cotton it is known as the bollworm, and in corn it is the corn earworm. It is a serious pest of all three crops and also occurs on hundreds of other plants, including many other row crops, vegetable crops, ornamental plants and weeds. These caterpillars can tolerate an amazingly large array of secondary plant chemicals.

Most of our serious caterpillar pests are the larvae of moths rather than butterflies. There are a few butterfly species whose caterpillars are pests, but this list is small. Imported cabbageworm is one example of a pest butterfly species. Black swallowtail butterfly is arguably another, but this depends on whether you are an herb gardener or a butterfly gardener! Would the monarch butterfly caterpillars that defoliated the milkweed plants in the city park butterfly garden be considered pests?

Caterpillars don’t only damage plants by eating leaves; some species cause damage in other ways. Pests such as tomato fruitworms and pickleworms bore directly into the fruit, while pests such as squash vine borers and peach tree borers bore into the stem or trunk. There is even a caterpillar that bores into the trunks of hardwood trees. It is called the carpenterworm.

Newly hatched caterpillars, like these cross-striped cabbageworms, often leave telltale “windowpanes” in leaves where they are feeding. (One-quarter inch)

Gardeners are often surprised by how quickly serious caterpillar damage can occur. “My azaleas looked fine when we were grilling in the backyard Saturday. Now it’s Tuesday, the azaleas don’t have any leaves on them, and we have all these big black and white caterpillars crawling around!” This same phenomenon occurs on many other ornamental and vegetable plants. Small, newly hatched caterpillars eat very little and their feeding often goes unnoticed, but large caterpillars that are almost ready to pupate can eat a lot in a short time. Many caterpillars take in 80 to 90 percent of their total food consumption in the last two or three days of their life – when they become the caterpillar equivalent of teenagers. This is why heavy caterpillar damage often seems to appear overnight.

For an article on how to control damage from these munching machines check out the follow-up article, Control Caterpillar Pests.

A version of this article appeared in an April 2012 print edition of State-by-State Gardening.
Photography courtesy of Blake Layton.

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